Proper lubrication is essential to engine life. When engine oil is continuously exposed to high temperatures, high pressure and an oxidizing (combustion) environment, the oil will deteriorate and lose its lubricating effectiveness. To protect the engine the deteriorated oil should be changed.
Analytical techniques in laboratories, accurate but very time consuming, are used to examine the oil's condition. Other methods, such as measuring the dielectric constant change in the oil or recording the thermal history of the oil have been used for monitoring the oil's condition. These methods require the use of the same engine oil or assuming no engine malfunctions throughout the whole measurements.
Using two-electrode structure sensor to measure the electrochemical activities or to measure the conductivity changes of chemically modified oil have been tried by others. The selectivity of such prior structures is not sufficient to differentiate between a new oil and deteriorated used oil, and are thus unacceptable. This is because the reactants quantities in new and used oils are high. Both oils have chemically active reactants in them; such as detergents and ZDP (zinc dialkyldithiophosphate) in the new oils and acidic oxidation products and blow-by contaminants in the used oils. Using a smooth electrode sensor, the current collected from measuring both new and used oils are high. Hence, an ordinary two-electrode sensor, either in "parallel plate" or interdigitated structure, has difficulty in distinguishing between new and degraded oils of the same type because all electroactive species are collected with near equal efficiency.
As the oil is degraded, both the physical and chemical properties of the oil will change. Monitoring the changes, it is possible to correlate the remaining oil life with these changes. However, different brands of new engine oils have different additive packages in them. It is very difficult to find a key parameter which changes in the same degree when the oil is no longer effective to protect the engine. For example, as was described in U.S. Pat. No. 4,733,556, issued Mar. 29, 1988, entitled "Method and Apparatus for Sensing the Condition of Lubricating Oil in an Internal Combustion Engine", the dielectric constant of an oil was used as the monitored parameter because it changes as the oil degraded. It is known that the dielectric constant of different brands of oils differ from each other. Therefore, it is difficult to find a dielectric constant value at which all brands of oil are definitely bad. Another prior method of evaluating oil is to monitor the viscosity of the oil. Unfortunately, the viscosity of the oil changed drastically only after some important additives were consumed. Hence, it is not practical to use oil viscosity to determine the oil conditions.
Oil lubricants have been used to lubricate and cool components of operating machinery. Often, as the oil lubricant performs its functions it undergoes thermal-oxidation degradation. It is also known that the addition of additive packages to the lubricant enhance and prolong the lubricant's useful life. However, after extended use, the additives of the lubricant are consumed and the useful life of the lubricant ends. Extended use of the lubricant beyond its useful life results in excessive component wear and eventual failure of the machinery. Naturally, it would be desirable to determine the point at which a lubricant's useful life ends so that it may be discarded and replaced with a new lubricant to insure the continued, safe, non-damaging operation of the machinery. Although mechanisms for an evaluating properties of lubricants have been known, such as that disclosed by Kauffman in U.S. Pat. No. 4,744,870, a qualitative sensing mechanism, particularly useful for automotive lubricants, has heretofore been unknown. Further, Kauffman's mechanism requires the mixing of a lubricant, solvent, organic base and electrolyte to produce an analysis sample that is measured outside of the engine.
Oil sensors having two electrodes in an interdigitated pattern are known. In operation, these oil sensors are immersed in an engine oil, and a saw-tooth AC voltage is applied to the electrodes. An electrochemical current with its magnitude depending upon the oil condition can be collected by the sensor. Normally, the use of an interdigitated two-electrode structure sensor has associated problems in differentiating between new and used oil because both oils have large quantities of chemically reactive species that produce higher sensor current or higher voltage when the sensor current is converted to a voltage output through an electronic circuit.
This invention describes how to solve the selectivity problem of prior art oil sensors. A feature of the present oil sensor is that the electrode surfaces are processed to make them rough.